Section 5 states that the IANA has set up the registry, but does not give any hints as to how to find it. A link to its location would be handy. I presume the correct location is http://www.iana.org/assignments/ftp-commands-extensions/ftp-commands-extensions.xml
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A a practice, we do not put URLs for registries into RFCs. Not an appropriate use of the errata system, in any event.